This section provides background information related to the present disclosure which is not necessarily prior art.
Power factor of an alternating current (AC) electric power system is defined as the ratio of the real power to the apparent power flowing to a load, and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf=50% pf). In an electrical power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in a power distribution system, and necessitate the use of large instruments and correspondingly large interconnecting cables. Hence, there is always a need for power supplies with improved power factor.
Power Factor Correction (PFC) allows the distribution of power at maximum efficiency. There are two types of PFCs viz., active PFC and passive PFC. Active PFC uses a circuit to correct the power factor and is able to generate a theoretical power factor of over 95%. Active PFC also markedly diminishes total harmonics, automatically corrects the AC input current, and is capable of handling a full range of input voltages. Passive PFC uses a capacitive filter at the AC input to correct poor power factor. Passive PFC requires that the AC input voltage be set manually and also does not use the full energy potential of the AC line.
The Total Harmonic Distortion (THD) of a signal is a measurement of the harmonic distortion present in a system and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Lesser THD allows a more accurate reproduction of signals by reducing harmonics added as a result of the electronic components present in the system. Hence power system designs must take into consideration the THD performance of the system.
Semiconductor switches are used in a power factor correction (PFC) system to control the flow of power to a load. Typically, integrated circuits (ICs) or digital signal processors (DSPs) are used to control the duty cycles of the semiconductor switches.
A prior art power factor correction (PFC) system 100 that uses a diode bridge rectifier and PWM (pulse width modulation) control using resistive current sensing is illustrated in FIG. 1. FIG. 2 illustrates a prior art bridgeless boost power factor correction (PFC) system 200 with PWM (pulse width modulation) control using only one current sensing resistor.
Various techniques have been used to provide an improved method of current sensing in a PFC circuit.